Conventionally, so-called curable pressure-sensitive adhesives have been proposed which provide operating simplicity unique to pressure-sensitive adhesives, ensure safety due to exclusion of volatiles, and exhibit bond strength and film strength comparable to those of adhesives.
For example, Japanese Patent Laying-Open No. Hei 2-272076 discloses a curable pressure-sensitive adhesive tape using a heat-curable pressure-sensitive adhesive comprised of a photopolymerizable composition containing an acrylate monomer and an epoxy resin. According to this reference, the photopolymerizable composition is first treated to allow the acrylate monomer alone to undergo polymerization to provide a pressure-sensitive adhesive tape. The reference describes that adherends are combined together using this pressure-sensitive adhesive tape and then heating is applied to cure the epoxy resin so that the sufficient bond strength is obtained.
However, in the method disclosed in Japanese Patent Laying-Open No. Hei 2-272076, heat is utilized to cure the epoxy resin and develop the bond strength. This prevents application of this method to adherends made of plastics or other materials which are poor in heat resistance, thereby limiting the material types of the adherends to be combined.
Japanese Patent Kohyo No. 5-506465 discloses a pressure-sensitive adhesive containing a free-radically photopolymerizable component such as an acrylate monomer, a cationically photopolymerizable component such as an epoxy compound, and an organometallic complex salt initiator. This pressure-sensitive adhesive has been proposed to improve adhesive strength. In the production of the pressure-sensitive adhesive, irradiation is carried out to polymerize both of the aforementioned free-radically photo-polymerizable component and cationically photopolymerizable component. That is, the pressure-sensitive adhesive results from the simultaneous inducement of the free-radical polymerization and cationic polymerization.
This pressure-sensitive adhesive is thus tailored such that those polymerization reactions complete while it is shaped, for example, into a sheet to impart sufficient sheet strength thereto. Although the pressure-sensitive adhesive exhibits excellent adhesive strength when joining adherends, no further improvement in bond strength thereof is expected from additional application of energy such as in the form of heat or light.
On the other hand, epoxy adhesives have achieved wide use in combining various members because they have the ability to bond a wide range of materials including metals, plastics and glasses and, when cured, exhibit improved resistance to creep, light, water, heat and chemical attack, as well as increased bond strength (“New Epoxy Resin”, edited and written by Hiroshi Kakiuchi, Shoko-do, published in 1985).
However, such epoxy resin adhesives are generally used in the liquid form. This usage has sometimes led to uneven coating of epoxy resin adhesives or bleeding of adhesives if applied excessively, which provides poor appearance at a joint edge. The liquid form of the epoxy resin adhesives has also made it difficult to recoat it on the once applied surface. Further, they are generally supplied in two parts, a base resin and a curing agent. This limits a mixing ratio thereof and the occasional deviation therefrom has caused adhesive defect.
To solve the above-described problems, an epoxy resin adhesive having a shaped sheet or film form has been proposed (Japanese Patent Laying-Open No. Sho 60-173076). However, such a sheet-form epoxy adhesive exhibits a high degree of elastic modulus at ordinary state and a low degree of initial adhesion and thus lacks a temporary holding capability. This lowers workablility during a joining operation, which has been a problem. Also because the sheet-form epoxy adhesive shows insufficient adhesion to adherends, severe curing conditions such as with a high-temperature or high-pressure press are required to combine the adherends. This has made it inapplicable to adherends which have the inability to endure under such curing conditions.
The inventor of this application and others have previously proposed a photopolymerizable composition utilizing two different polymerization modes (Japanese Patent Laying-Open No. Hei 9-279103). That is, a photo-polymerizable composition has been proposed which contains a free-radically polymerizable monomer such as an acrylic monomer, a free-radical polymerization catalyst, an epoxy-containing compound, and a cationic polymerization catalyst that initiates curing of the epoxy-containing compound. This composition is first exposed to a radiation to activate the free-radical polymerization catalyst. The subsequent polymerization of the free-radically polymerizable monomer results in the production of a tackifying polymer. The resultant is then shaped into a sheet. In use, this sheet is exposed to a radiation that activates the cationic polymerization catalyst. The subsequent curing of the epoxy resin results in obtaining sufficient bond strength.
However, if the curing of the epoxy resin is to be achieved, the sheet must be aged for a long period of time after exposure to a radiation. This has been a problem.
Also, the inventor of this application and others have previously proposed a curable pressure-sensitive adhesive by which adherends can be combined with each other under the curing conditions that little limit the types of the adherends used (Japanese Patent Laying-Open No. Hei 10-120988). In this reference, a curable pressure-sensitive adhesive sheet is disclosed which contains an acrylic polymer, an epoxy resin and a cationic photoinitiator. When this curable pressure-sensitive adhesive sheet is exposed to a radiation, the cationic photoinitiator is activated to initiate curing of the epoxy resin. Adherends can be joined firmly to each other simply by irradiating the curable pressure-sensitive adhesive sheet either before or after it is applied to the adherend. The heat resistance of the adherends used does not matter here.
The inventor of this application and others have also found that the curing reaction is caused to proceed as soon as the curable pressure-sensitive adhesive sheet is exposed to a radiation, its elastic modulus starts to rise after a while, and its bond strength arrives at a sufficient level after about 24 hours at room temperature.
Bond strength evaluation revealed the sufficiently increased shear strength but the occasional occurrence of insufficient peel strength.
In view of the current state of the art as described above, the present invention is directed to provide a photocurable composition which utilizes two polymerization modes, can be cured utilizing one of the polymerization modes, imposes little limitation on the type of the adherend used, has a long work life, and when cured exhibits improved bond strength, particularly in peel strength.
It is another object of the present invention to provide a photocurable pressure-sensitive adhesive sheet which is made by sheeting the aforementioned photocurable composition and when cured exhibits improved bond strength, particularly in peel strength and also provide a joining method using the sheet.
It is still another object of the present invention to provide processes for production of the aforementioned photocurable composition and photocurable pressure-sensitive adhesive sheet.